I used to own this exact lens, and had a similar experience after a weeks-long road trip. Apparently all the vibrations from travel shook some things loose. While I don't know that your lens has the same exact problem, I was able fix mine (and sell it as well). First, I removed the lens mount, just the chromed bayonet which is held on the back of the lens by three screws. Then I tightened some screws that hold the barrel to the rear assembly (the piece that includes the aperture ring), and then put the mount back on. As far as I remember, my repair did not involve any complex work; I did not have to remove the aperture mechanism or anything else. The screws that I tightened should be easily visible when you remove the mount. Good luck!